1. Field of the Invention
This invention relates to a two stage process for the production of aromatics having a particularly high proportion of benzene from C.sub.2 to C.sub.10 aliphatic hydrocarbons.
2. Background Information
It is known to use zeolites in the preparation of catalysts for the production of aromatic hydrocarbons from aliphatic hydrocarbons by passing the aliphatic hydrocarbon over the catalyst at an elevated temperature in the liquid or vapor phase. Zeolites of various types have been suggested for the preparation of such catalysts, e.g., ZSM-type zeolites containing any of various cations such as hydrogen, ammonium or metallic cations, e.g., gallium, or containing any of various metal oxides impregnated on its surface, e.g., a gallium oxide. However, while the yield of "BTX" aromatic fractions, i.e., consisting of benzene, toluene, xylenes and ethylbenzene, using the foregoing processes, may be relatively high, the yields of benzene alone, which is a particularly valuable feedstock for chemical processes, are fairly low. Thus, any means to increase the yield of benzene from aliphatic feedstocks, e.g., raffinates from aromatics extraction processes, is highly desirable.
3. Information Disclosure Statement
The following information is disclosed in accordance with the terms of 37 CFR 1.56, 1.97 and 1.98.
U.S. Pat. Nos. 3,756,942 and 3,760,024, disclose the preparation of aromatic compounds from aliphatic compounds using as catalyst a ZSM-type zeolite in which at least some of the original cations have been replaced by hydrogen, ammonium or metallic cations, or impregnated with a metallic component as described previously.
U.S. Pat. Nos. 4,180,689, 4,304,686, 4,350,835 and European Patent Specification Publication No. 50,021 each discloses a process of converting aliphatic compounds to aromatics utilizing as catalyst a zeolite, e.g., ZSM-5, containing gallium in impregnated or cation exchanged form.
The foregoing and similar prior art disclosures emphasize the conversion of aliphatics to aromatics broadly, or, in some cases, to the "BTX" fraction of aromatics composed of benzene, toluene and C.sub.8 aromatics, to be used, for example, as a high octane gasoline blending component. However, there is generally no suggestion in such disclosures of the desirability of maximizing the production of benzene, specifically, e.g., for use as a chemical feedstock.
U.S. Pat. No. 4,560,820 teaches a process of de-alkylating alkylaromatic hydrocarbons, e.g., toluene to produce benzene, using intermediate pore size zeolites substantially free of acidity. However, there is no suggestion in this patent of the application of the process to a feedstock which already contains a substantial proportion of benzene.
Hughes et al., "Aromatization of Hydrocarbons Over Platinum Alkaline Earth Zeolites," in New Developments in Zeolite Science and Technology, Vol. 28, pp. 725-732, teach platinum-containing alkaline earth exchanged alkali metal Zeolite L, e.g., platinum containing barium-exchanged potassium zeolite L (Pt/BaKL), used as a catalyst for the aromatization of paraffins. There is, however, no disclosure by this reference of the conversion of any feedstock which already contains a substantial proportion of aromatics such as benzene.